The invention relates to a current sensor comprising a ring core made of non-magnetic material, a primary winding constituted by a conductor running through said core, a secondary winding constituted by a turn wound on the core and a magnetic shield externally surrounding the core with the secondary winding.
Non-magnetic sensors of the kind mentioned are not sensitive to magnetic saturation phenomena, but they deliver a relatively weak signal. They are commonly used in electronic monitoring or protection systems, in particular in low voltage, but their use in medium and high voltage gives rise to problems of noise, the level of which can reach that of the signal.
The object of the present invention is to enable a non-magnetic current sensor to be produced, having a low noise level, particularly in medium or high voltage installations.
The sensor according to the present invention is characterized by the fact that it comprises an electrostatic shield made of non-magnetic conducting material, fitted between the primary conductor and the non-magnetic core with the secondary winding.
The external magnetic shield provides shielding which enables external interference, notably any external noise induction, to be suppressed. It has however been found that this external shielding is insufficient and that the signal delivered by the sensor varies with the voltage applied to the sensor. By adding an electrostatic shield according to the invention, this disturbance of the signal delivered by the current sensor can be eliminated and an accurate measurement signal be obtained, even in high voltage installations. The study of this phenomenon shows that it is caused by the capacitance existing between the primary conductor and the external layer of the secondary winding, which explains the efficiency of the electrostatic shield.
It is advantageous to connect the electrostatic shield to the earth or ground to avoid any electrical potential in the measurement zone.
The magnetic shield is preferably a cylindrical shell made of a high magnetic permeability material, inside which is housed the ring core supporting the secondary winding. The conducting electrostatic shield has a very low permeability and can be made of copper or aluminium or any other suitable material. Various shield structures are conceivable, notably a rigid part fitted partially or totally around the core with its winding or a conductive band wrapped around the core and its winding or again a single conducting layer, for example a coat of paint or a conductive coating.